1. Field of the Invention
The present invention is generally related to an apparatus for determining bolt load and more particularly to an apparatus for remotely monitoring bolt load during and after torquing of the bolt.
2. General Background
In many situations, it is necessary to determine bolt load during assembly operations to insure that parts are joined together within the stress limits for the intended use. In applications where the bolt load is critical, the use of a torque wrench is generally unacceptable due to varying amounts of friction between the bolt and nut, structure, or washer. Devices for indicating the load on a bolt or fastener which applicant is aware of include the following.
U.S. Pat. No. 4,846,001 discloses a piezoelectric sensor permanently coupled to the upper surface of a fastener, contacts electrically engageable with the sensor, and an electronic measurement device responsive to electronic differential signals of the sensor to determine the tensile load of the fastener when stressed longitudinally.
U.S. Pat. Nos. 4,686,859 and 4,676,109 disclose a bolt modified to provide a load indicating fastener wherein a pin is positioned within an axial bore in the bolt so the head of the pin is coplanar with the bolt head. A load indicating device determines bolt load during installation by the use of a differential displacement measurement device to measure the height differential between the upper surfaces of the bolt head and the pin.
U.S. Pat. No. 4,333,351 discloses a method and apparatus for determining the residual tension in a bolt or stud in a joint assembly. A tensioner is used to pull the bolt or stud in a previously tightened joint and an ultrasonic extensometer is used to determine the residual force or tension in the bolt or stud.
U.S. Pat. No. 4,709,182 discloses the use of a vibrator/sensor unit to cause longitudinal oscillation of a bolt to its natural frequency to aid in loosening or tightening the connection to a predetermined value.
U.S. Pat. No. 3,759,090 discloses an ultrasonic extensometer for measuring the elongation of a bolt tightened against a structure. A transducer magnetically coupled to the head of the bolt generates pulse signals into the bolt.
Although the known art provides devices for measuring bolt load, a need exists for a device that is capable of remotely torquing a bolt while simultaneously and consistently indicating the load on the bolt. Much of the known art also requires the use of a modified bolt to determine bolt load. This results in increased costs. Therefore, it can be seen that there is a need for a device that does not need separate torquing and measuring tools and can use the standard bolt intended for the application.